Cheapest priced copy (Oath of the Gatewatch): $4.99 at Card Kingdom, near mint, their price list as of .
Stoneforge Masterwork 
Artifact — Equipment · Rare · Oath of the Gatewatch · #166
Equipped creature gets +1/+1 for each other creature you control that shares a creature type with it.
Equip {2}
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $4.99 to $8.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Oath of the Gatewatch | 166 | Rare | $4.99 | $8.99 |
| Oath of the Gatewatch Promos | 166s | Rare | — | $7.99 |
2 rulings from Wizards of the Coast
- 22 January 2016The bonus is only +1/+1 per applicable creature, even if it shares more than one creature type with the equipped creature. For example, if the equipped creature is a Human Shaman and you control a Human Warrior, a Goblin Shaman, and another Human Shaman, the equipped creature will get +3/+3.
- 22 January 2016The bonus will change as the creatures you control change. This may cause the equipped creature to lose toughness and die because of damage dealt earlier in the turn.